Determining the f ratio 11/16/2010. Incubate seawater in the presence of trace 15

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1 Plankton production is supported by 2 types of nitrogen: 1) new production supported by external sources of N (e.g. NO 3 and N 2 ), 2) recycled or regenerated production, sustained by recycling of N. Assumptions: 1) N 2 fixation is low 2) Steady state system 3) Euphotic zone nitrification is low The f ratio f = (VNO 3 )/ (VNO 3 + VN R ) Note N R includes regenerated forms of N uptake (historically thought to include urea and NH 4+ ) Mth Mathematical ti ldescription linking new N 2 regenerated NH + 4 Biological production production and organic matter export. At steady state, nitrogen input is balanced by nitrogen export. NO 3 export Under steady state (i.e. nitrate input balanced by export/grazing loss), if export is less than input, biomass accumulates. This biomass must eventually be exported to keep the system in steady state. NO 3 NO 2 NH 4 + N export Why does this generalization apply to the open sea but not near shore environments? Determining the f ratio 15N 15N 15N Incubate seawater in the presence of trace 15 NO 3, 15 NH 4+, and sometimes 15 N urea Calculate NO3, NH4+, and DON uptake What makes this difficult for the oligotrophic ocean? Net Uptake 15N 1

2 Autonomous sensing of nitrate 2

3 Not all new nutrients are introduced to the euphotic zone from below Johnson et al. (2010) Nature Atmospheric deposition (both dry and wet) can form an important source of nutrients. Advection: lateral input of nutrients N 2 fixation Assimilation of N by N 2 fixation N 2 fixation is the primary mode of nitrogen introduction to marine and terrestrial ecosystems. N 2 fixation converts N 2 to NH 3 ; exclusively prokaryotic process Requires significant energy expensive 3

4 At Station ALOHA, N 2 fixation appears to contribute ~30 84% of new production Each annual cycle at HOT and BATS has significant Dissolved Inorganic Carbon (DIC) drawdown, but not enough nitrate is present in surface water to support growth. 15 N particulate N export ( ) Year NO 3 N 2 from Karl et al. in Fasham, Ocean Biogeochemistry Let s look at dissimilatory nitrogen transformations Oxidized N Energy to be gained in oxidation Global estimate of N 2 fixation based on N DIC drawdown in NO 3 depleted warm waters is equivalent to Pg C yr 1 Reduced N (Sarmiento & Gruber, 2006) 4

5 Dissimilatory nitrogen transformations NH 2 OH NO N 2 O Nitrification: NH + 4 NO - 2 NO - 3 Denitrification: NO 3- NO 2- NO N 2 O N 2 Anammox: NO 2 + NH 4+ N 2 + 2H 2 O Nitrification Biological oxidation of NH 3 to NO 3- using oxygen as terminal electron acceptor. Two step process; ammonia oxidation followed by nitrite oxidation; both reactions yield energy. NO 2- serves as an important intermediate; incomplete nitrification also yields N 2 O. Aerobic regeneration of nitrogen Complete decomposition of organic matter Degradation of organic N to ammonium occurs during heterotrophic metabolism. Nitrification is a 2 step process that is mediated by different groups of microbes. The first step (termed ammonium oxidation) oxidizes NH 4+ to NO 2-, and the second step converts NO 2- to NO 3-. (CH 2 O) 106 (NH 3 )16H 3 PO O 2 106CO H 2 O +16HNO 3 + H 3 PO 4 Multi-step process. First step is the breakdown of amino acids to NH 4+ ; this process is mediated by heterotrophic microorganisms 2NH O 2 2NO H + + 2H 2 O 2NO - +O NO 3 These reactions yield energy (but not much ) Nitrification: predominately mediated by chemoautotrophic microbes (best studied are Nitrosomonas and Nitrobacter) 5

6 Recent isolation and cultivation of an abundant archaeal ammonium oxidizer Archaea 20-40% of total picoplankton in meso- and bathypelagic waters 0 Let s look at dissimilatory nitrogen transformations 1000 Depth (m) 2000 Pacific Atlantic Oxidized N NO NO 2 - ( mol L -1 ) Energy to be gained in oxidation Reduced N (Sarmiento & Gruber, 2006) 6

7 Denitrification The reduction of NO 3- and NO 2- to N 2 during heterotrophic respiration of organic matter. Occurs predominately in anaerobic or suboxic environments. C 106 H 175 O 42 N 16 P NO 3-106CO N 2 + H 3 PO H 2 O NO 3- and NO 2- are used as terminal electron acceptors during heterotrophic respiration. Anaerobic ammonium oxidation (anammox) NH 4+ + NO 2-2N 2 + 2H 2 O Anaerobic ammonium oxidation Major source of N 2 gas (along with denitrification) Anoxic sediments, marine water column, and sewage wastewater Mediated by Planctomyces Oxygen concentrations along the 26.9 kg m -3 isopycnal surface (~500 m in the N. Pacific) High productivity in surface water due to upwelling of nutrients. High organic matter High organic matter flux depletes O 2 concentrations below the euphotic zone. Chlorophyll distributions 7

8 N input by N 2 Losses of N by fixation denitrification N 2 Atmosphere N 2 N 2 O Photosynthesis Organic Bacterial matter degradation NH + 4 Aerobic Suboxic Detritus Bacterial degradation NH 4 + Nitrification NO 2 - NO 3 - Denitirifcation N 2 N 2 O N 2 O 1 Tg = g Global Nitrogen Budget Process Nitrogen Flux (TgN yr -1 ) Sources Pelagic N 2 fixation Benthic N 2 fixation River input (DON) River input (PON) Atmospheric deposition TtlS Total Sources Sinks Organic N export 1 Benthic denitrification Water column denitrification Sediment burial N 2 O loss to atmosphere 4 2 Total Sinks